1. Field of the Invention
The present invention relates to an object measuring apparatus using a so-called fringe scanning interference method for measuring the configuration or the like of an object by performing fringe scanning wherein a phase difference between a reference light and a light from an object is caused to change with time.
2. Related Background Art
There is known an object measuring apparatus using lightwave interference as shown in FIG. 3.
In FIG. 3, reference numeral 1 represents a laser light source for generating a coherent light beam, 2 a beam expander optical system, 3 a polarized light beam splitter, 4a and 4b a quarter wavelength plate, 5 an electro-optical element, 6 a power source for applying a voltage changing with time to the electro-optical element 5 to change the degree of optical anisotropy of the electro-optical element 5, 7 an object to be measured, 8 a reference mirror, 9 a lens, 10 a photodetector constructed of a plurality of light receiving elements such as CCDs disposed two-dimensionally, 11 a polarizing plate, and 30 a control circuit.
The diameter of a light beam from the laser light source 1 is made longer by the beam expander optical system 2, and applied to the polarized beam splitter 3. In accordance with the polarization direction, the incident light beam is transmitted to the quarter wavelength plate 4a or reflected to the electro-optical element 5 and quarter wavelength plate 4b. The transmitted beam component is changed to a circularly polarized wave by the quarter wavelength plate, reflected by the object 7 to be measured, again applied to the quarter wavelength plate 4a with its polarization plane having been rotated by 90 degrees from that when it was first applied thereto, and returned to the polarized beam splitter 3. This beam from the object 7 to be measured is therefore reflected by the polarized beam splitter 3 in this time downward as viewed in FIG. 3.
On the other hand, the reflected beam component having a polarization plane, e.g., in the direction vertical to the drawing surface, is transmitted through the electro-optical element 5 having a properly directed optical axis because of the refractive index for an extraordinary light beam, and is applied to the quarter wavelength plate 4b, and reflected by the reference mirror 8. The reflected beam is again applied to the quarter wavelength plate 4b with its polarization plane having been rotated by 90 degrees from that when it was first applied thereto, and then applied to the electro-optical element 5. This reflected beam component therefore is transmitted through the electro-optical element 5 because of the refractive index for an ordinary light beam, and to the polarized beam splitter 3 to be directed downward as viewed in FIG. 3.
The reference light beam from the reference mirror 8 and the light beam from the object 7 are superposed together and applied via the properly disposed polarizing plate 11 and lens 9 to the photodetector 10. Both the light beams interfered with each other at the interface of the polarized beam splitter 3 pass through the polarizing plate 11 so that they are detected by a plurality of light receiving elements of the photodetector 10 as a distribution of interference fringes. An output of the photodetector 10 is supplied to the control circuit 30.
The degree of optical anisotropy of the electro-optical element changes considerably with an applied voltage changing with time. The ordinary and extraordinary refractive indices change (as the former increases, the latter decrease, or vice versa) so that the phase of the reflected beam component transmitted two times through the electro-optical element is caused to change. The phase difference between the reference light beam and the light beam from the object 7 therefore changes so that the interference fringes on the photodetector 10 are caused to move. The moving interference fringe data are picked up in synchronization with the operation of the power source 6 by which the optical anisotropy of the electro-optical element 5 is changed, and analyzed by a computer within the control circuit 30 to thereby measure the flatness, configuration or the like of the object 7 or the amount of motion of the object 7. This measurement method is called a fringe scanning interference method. According to this method, errors caused by atmospheric fluctuation, noises or the like can be minimized and a more precise measurement can be realized. The details of this method is described, e.g., in the document by S. Yokozeki, K. Patorski and K. Ohnishi Optics Commun., 14 (1975) 401, in "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses": Applied Optics, Vol. 13, No. 11, Nov. 1974, and in other documents.
There is also known another arrangement without the electro-optical element 5 wherein the phase difference between the reference light beam and the light beam from the object 7 is changed by moving the reference mirror 8 with a piezoelectric element for example This arrangement will be described with reference to FIG. 4 wherein identical reference numerals denote like elements to those shown in FIG. 3.
In FIG. 4, reference numeral 12 represents a piezoelectric element 12, 6 a power source for applying a voltage changing with time to the piezoelectric element 12 to expand or compress it in the optical axis direction, 7 an object to be measured, 13 a reference mirror fixedly attached to the piezoelectric element 12, 9 a focussing lens, 10 a photodetector constructed of CCDs or the like, 11 a polarizing plate, and 30 a control circuit.
The diameter of a light beam from the laser light source 1 is made large by the beam expander optical system 2, and applied to the polarized beam splitter 3. In accordance with the polarization direction, the incident light beam is transmitted to the quarter wavelength plate 4a or reflected to the quarter wavelength plate 4b. The transmitted beam component is changed to a circularly polarized wave by the quarter wavelength plate 4a, reflected by the object 7 to be measured, again applied to the quarter wavelength plate 4a with its polarization plane having been rotated by 90 degrees from that when it was first applied thereto, and returned to the polarized beam splitter 3. This beam from the object 7 to be measured is therefore reflected by the polarized beam splitter 3 this time downward as viewed in FIG. 4.
On the other hand, the reflected beam component is changed to a circularly polarized wave by the quarter wavelength plate 4b, reflected by the reference mirror 13 moving in the optical axis direction, again applied to the quarter wavelength plate 4b with its polarization plane having been rotated by 90 degrees from that it was first applied thereto, and then applied to the polarized beam splitter 3. The reflected beam component is then transmitted this time through the polarized beam splitter 3 to be directed downward as viewed in FIG. 4.
The reference light beam from the reference mirror 13 and the light beam from the object 7 are superposed together and applied via the properly disposed polarizing plate 11 and lens 9 to the photodetector 10. Both the light beams interfered with each other at the interface of the polarized beam splitter 3 pass through the polarizing plate 11 so that they are detected with a plurality of light receiving elements of the photodetector 10 as the distribution of interference fringes. An output of the photodetector 10 is supplied to the control circuit 30.
Since the reference mirror 13 moves in the optical axis direction, the phase difference between the reference light beam and the light beam from the object 7 changes to thereby move interference fringes on the photodetector. The moving interference fringe data are picked up in synchronization with the operation of the piezoelectric element 12 by which the reference mirror 13 is caused to move, and analyzed, in the similar manner as the first mentioned prior art, by a computer within the control circuit 30 to thereby measure the configuration or the like of the object 7.
The above prior art is associated, however, with the following problems. Namely, if an object having a large area is to be measured, the diameter of a light beam must be made large correspondingly so that the electro-optical element for generating a reference light beam and the reference mirror must also be made large. This requirement lead to a necessity for a uniform characteristic of the electro-optical element over a certain area and a precise motion of a certain area of the reference mirror in the optical axis direction without any inclination thereof. Sophisticated and highly cumbersome techniques are required in manufacturing such an electro-optical element, piezoelectric element and reference mirror and in positioning the piezoelectric element and reference mirror, thereby resulting in high cost.
Particularly in the prior art shown in FIG. 3 wherein there is used the electro-optical element disposed in the optical path of the reference light to thereby give the light beam a phase difference while it reciprocally propagates the reference light optical path. If the light beam undergoes the extraordinary refractive index of the electro-optical element while it first passes therethrough, the reflected light beam thereafter undergoes the ordinary refractive index of the electro-optical refractive index while it again passes therethrough because the polarization plane has been rotated by 90 degrees (the reverse case is also subjected to the same phenomenon). Since the ordinary and extraordinary refractive indices change oppositely to each other, the phase change amount while the light beam again passes through the electro-optical element is cancelled out. It is therefore necessary to apply a large electric field to the electro-optical element in order to obtain a large phase difference.
Further, in the prior art shown in FIG. 3, as the characteristic of the electro-optical element 5 changes with temperature or the like, only the optical path of the reference light is influenced by the characteristic change so that the relative phase difference between the reference light beam and the light beam reflected from the object becomes less reliable. The resultant measured value is therefore less reliable which is susceptible to a change in external environment such as temperature change.